The art of lithography is well known and is practiced commercially in several fields including, for example, the electronic industry, newspaper industry, magazines, and the like. The field of lithography has been extensively written and commented upon in several books and articles. See, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 19, p. 140, John Wiley & Sons (1982); ibid, Vol. 20, p. 161 (1982); and Encyclopedia of Polymer Science and Engineering, Vol. 13, p. 373, John Wiley & Sons (1988).
A typical lithographic printing plate comprises a substrate surface, typically made of aluminum, silicon and the like, with a photosensitive layer on top. The preparation of the lithographic printing plate generally comprises cleaning the aluminum base, roughening it to make it suitable to receive the layers that go on top, anodically oxidizing it, followed by conditioning the surface and depositing the photosensitive layer on top. If defects occur in any of these steps, the quality of the printing plate suffers. Thus, for example, when the conditioning process is not of optimum quality, it affects the sensitivity to the background in the printed images, resulting in "toning". Copending patent application, Ser. No. 08/128,911, filed of even date herewith, describes an improved process for conditioning and improving the hydrophilicity of the plate.
The roughening ("graining") step is another area of concern in the lithographic industry. Traditionally roughening of aluminum plates is performed by electrochemical or mechanical process. U.S. Pat. No. 4,786,381 discloses an electrochemical process for modifying the surface of an aluminum plate. The mechanical process includes processes such as ball graining, brush graining, and the like.
There are several problems associated with known roughening processes. Thus, for example, in the ball graining process, high skills are required for the selection of the kind or the size of balls, control of water during the abrasion and the like. The brush graining process has some drawbacks too. In brush graining, generally the plate is brushed with a brush in a slurry comprising the roughening particles. It is difficult to obtain high roughness and scattering is easily formed on the coarse face by the wear of the abrasion brush being used. Further, there are problems that the surface of the aluminum is scratched by the strong friction between the brush and larger particles of the abrasion.
U.S. Pat. No. 4,183,788 discloses a brushing process for making lithographic plates wherein the lithographic base, aluminum, is grained with an aqueous slurry comprising unfused, platy crystalline alumina using a rotary brushing motion whose force is predominantly tangential to the surface of the base plate. This process has some disadvantages too. The alumina does not stay suspended in the aqueous slurry during use. There is a tendency for the particulates to settle down during use, necessitating the continuous use of pumps to keep the particles in a state of suspension.
In an effort to avoid such problems with alumina-containing slurry, silica (quartz) has been proposed as the graining medium. However, quartz is known to cause a health hazard, silicosis, particularly when it is present as fine particles. Furthermore, quartz particulates break down easily during use, necessitating continuous replenishment.
Thus, it would be desirable to have a graining process whereby good quality lithographic plates can be produced without the above-noted problems. It would be preferable if such graining processes result in fewer number of scratches on the substrate surface. It would also be desirable if the process has minimum breakdown of the particles or increased slurry stability and thus does not need an undue amount of replenishment during a run.
SUMMARY OF THE INVENTION
Applicants have discovered that employing an aqueous slurry comprising a mixture of alumina and quartz in a brush graining process surprisingly yields a highly improved lithographic substrate surface. The inventive slurry has good use life and does not need undue replenishment during use. Furthermore, and surprisingly, the inventive slurry comprising quartz and aluminum causes far fewer number of scratches on the substrate surface than a slurry comprising alumina without quartz under comparable conditions.
The inventive slurry generally contains about 5-95 weight %, preferably about 5-50 weight % and typically about 10-20 weight % of quartz in alumina. The alumina and quartz particulates have a mean particle size of about 1-20 microns, preferably about 1-10 microns and typically about 3-6 microns. While the graining may be carried out by the conventional method of using a plurality of brushes, in most case use of a single brush is sufficient.